Preparation of isopropanol of improved odor quality



Jan. 3, 1956 H. o. MOTTERN 2,729,632

PREPARATION OF ISOPROPANOL OF IMPROVED ODOR QUALITY Filed May 20, 1950 Pc t QQNdEN-mA-rN figgflol COLUMN 5 1% 1::-=-WAT5Q QIQ'JTiLt-AI 'ION AvCOLUMN 1 momma; ALCOHOL 65 M M Qbtorrle$ United States Patent i Researchand Engineering Company,

Delaware Application May 20, 1956, Serial No. tea-sis 4 Claims. .(Ci.Mil-641) assiguor to Essa a corporation of This invention relates to'acombination process for the production aud purification of isoprop lalcohol and articularly to the production of an iso ropyl alcohol orimproved odor quality in high yields, i. e., about 95%. Specifically,the invention relates to a process whereby isopropyl alcohol is-producedfree of malodorous impurities of the mercaptan and thioaldehyde type.More particularly this invention iscOnce'rned With a novel process forthe production of isopropanol free of the above-mentioned rnalodorousimpurities comprising a combination of steps involving (1) theproduction of a crude isopropanol by the sulfuricacid catalyzedhydration of propylene containing about 0.2 "to 5 wt. per cent,preferably 0.5 to 1 wt. per cent (based on total hydrocarbons) ofhydrocarbons, containing 4-'t'o ec'ar'boh atoms per molecule,particularly C4 hydrocarbons, said C4-Cs hydrocarbons consisting of atleast 50 wt. per cent C4-C6 monoolefins, and (2) the subsequentpurification of the crude isopropanol by water extractivedistillation.

.It is well known that alcohols, particularly those produced by theacid-catalyzed hydration of olefin hydrocarbons, possess. a distinct andapparently foreign odor, slightly penetrating and for the most partdisagreeable. While no attempt will be made to definitely assign thedisagreeable odor of alcohols prepared by olefin hydration to thepresence of any one or combination of chemical compounds, it can be saidwith reasonable assurance that theodor of crude alcohol depends to alarge extent on the quality of the olefin stream employed in the olefinhydration operation. Likewise, the odor of a refined alcohol depends toa large extent on the quality of the crude alcohol from which it isprepared.

Olefin'h'ydrocarbons, such as those produced by the cracking of mineraloils, contain variable amounts of compounds having an obnoxious odorparticularly sulfur compounds such as hydrogen sulfide, alkyl sulfidesand mercaptans. These materials present, even in minute amounts, in theolefin stream to the acid catalyzed hydration process are believed tocontribute to the obnoxious odor of the crude alcohol, since, whilesulfur alone has no smell, it is clear that in combination with otherelefn'ents it is a powerful odoriferous agent. The bad odor of alcoholshas also been attributed to the presence of so-called high-boilingpolymer products formed in side reactions during the alcohol process.The odor of the polymer products isfs'tren'g'thened by the presence ofany sulfur compounds dissolved therein, although the odor of some or thepure polymers themselves is -by no means pleasing to the olfactory"sense. Atypical analysis of a sample of the so-called polymer product,in this case the so-called propyl oil, resulting from the production ofisopropyl alcohol by the sulfuric acid catalyzed hydration of propylene,is as follows:

55 wt. per cent sec-heptanol (B. P. 137140 C.) 21 wt. per centsec-octanol (B. P. 160-165 C.) 2 wt. per cent C-z ketone (B. P. 131 C.)13 wt. per cent hydrocarbons (B. P. above 180' C.) 9 wt. per cent ethers(B. P. above 160 C.)

traces of sulfur compounds.

-. 2,729,682 Patented Jan. 3, 1956 2 The composition of the propyloil orso-called high-boiling polymers obtained during the concentration ofdilute crude isopropanol varies according to the point from which thealcohol containing it is withdrawn in the concentrating tower, and theodor likewise varies. Cuts can be identified with an odor of camphor, ofmenthol, etc. It has been reported also that the presence of nitrogencompounds likewise contributes to the odor of alcohols.

The odor imparted to isopropylalcohol by the propyl oil impuritiles is ahydrocarbon type odor, sometimes referred to as butyl odor. This type ofodor has been found to be effectively removed by a number of methodssuch as by careful and repeated conventional fractionations, butparticularly by the water extractive distillation method as describedand claimed in Serial No. 24,626 filed May 1, 1948, how U. S. Patent No.2,638,440, in the name of William M. Drout, Jr. et a1., and assigned toapplicants assignee.

However, there is a second type of odor which is fugitive in natureandwhich may best be characterized as a mer'c'aptan or th'ioaldehyde'odor, and which is attributed to the presence of low-boiling sulfurcompounds such as mercaptans or thio'aldehydes which appear to bedecomposition products of higher-boiling impurities which break up underalcohol purification distillation conditions. It has been foundthat thissecond type of odor is particularly noticeable when the alcoholproduction process is operated under upset regenerator conditions orwhen the purification 'proce'ssis operated under upset concentratorconditions. Whenthe capacity of the regenerator or concentrator isexceeded some isopropyl alcohol creeps down into the reboiler associatedwith the regenerator or concentrator wherein a high tem erature prevailsand Where a particular concentration of sulfuric acid exists. Underthese conditions it is felt that the sulfuric acid present inthe-reboiler acts as a reducing agent on the isopropyl alcohol whichdecomposes into lower-boiling impurities, particularly mercaptan and/o1-thioaldehyde com ounds which are volatile. These impurities go overheadwith the alcohol from the re enerator. Subsequently, in the concentratorthey are not all removed with the low boilin'g overhead, but remain intraces of parts per million in the isopropyl alcohol product which isiremoved as a top Sidest'reatn from the conceutrator. Likewise, if theconcentrator conditions are upset, the same decomposition occurs and theroblem becomes aggravated. It has'also been theorized that this secondtype of odor is due to the decomposition of suifuri'zed esters which areformed minute amounts during the propylene absorption in sulfuric acid.These esters are thought to decompose either thermally or by hydrolysis.In commercial operations this type of odor has been termed recycleodoralthough the odor is no way attributable to recycle operationsencountered in the production of isopropanol.

Furthermore, there are indications that some of the compounds givingrise to mercaptah odor are rather strongly bound in theaqu'eous alcoholproduct in such a manner that they tend to concentratealoh'g with thealcohol during such operations as heads removal, water extractivedistillation and even regular distillation for alcohols finishing.However, the-nature of the chemical 'reac tions tending to bind these:m-alodorou's compounds issuch that unstable addition compounds resultand the malodorous compounds are liberated and volatilized to someextent during all alcohol finishing operations of the types described.Forinstance, in typical alcohol finishing operations the followingproblems are encountered. Crude alcohol atter passing through the headsoperation to remove ether and other low boilers, is fed to a finishingcolumn in which certain low-boiling materials are taken overhead with apurified alcohol taken as a sidestream.

Materials contributing to recycle odor appear to be lower boiling thanthe alcohol proper but are nevertheless not completely removed from thealcohol sidestream. This result is believed to be due in large part tothe fact that decomposition reactions liberating low-boiling malodorouscompounds occur during the distillation, allowing these malodorouscompounds to pass overhead. Similar difiiculties are encountered whenapplying water extractive distillation to the same crude alcohol. Infact, Water extractive distillation appears to concentrate the compoundsgiving rise to mercaptan odor along with the alcohol so that, in thefinal alcohol finishing operation, an alcohol product high in mercaptanodor is obtained. This seems to be particularly true of isopropylalcohol prepared by the so-called weak acid method as described below.

Crude isopropyl alcohol is produced by the sulfuric acid catalyzedhydration of propylene by the weak acid method or the strong acidmethod. In the former process a relatively clean ca-hydrocarbon streamcontaining about 50-80 volume percent propylene is absorbed in sulfuricacid. The relatively clean Cs-hydrocarbon stream is a product of thecareful fractionation of a refinery Cs-stream so that it contains onlytraces to a maximum below 0.2 wt. percent of C4-olefius plusCoparafiins. Acid of approximately 60 to 80 wt. percent concentrations,preferably 70 wt. percent, is employed at about 170 F. and approximately250 p. s. i. g. pressure to form an extract comprising isopropyl sulfatewhich is partially hydrolyzed to alcohol during the absorption. Thehydrolysis is completed by dilution of the extract with water toapproximately 45 wt. percent acid strength and maintaining the extractat a temperature of about 190 F. for a period of about minutes residencetime. The hydrolysis products are then distilled in an alcoholregenerator wherein crude alcohol vapors are removed overhead andcondensed, and wherein spent sulfuric acid is recovered as bottoms forreconcentration and. subsequent re-use in the absorption process. Thecrude isopropyl alcohol contains approximately 30-60% by volume ofisopropyl alcohol, some irnpurities including hydrocarbons, isopropylether, acetone and unknown impurities, and the balance Water. The crudealcohol is condensed and cooled and is ready for the purification stageof the process.

In the strong acid process a propylene stream of relatively lowerpropylene content, i. e., about 30-60 volume percent propylene iscontacted under similar conditions with a stronger sulfuric acid, e. g.,acid of 80-95 wt. percent concentration, preferably 85-88 wt. percent,whereby an extract is formed as above. The extract goes through asimilar hydrolysis and stripping treatment to produce a crude isopropylalcohol as described for the weak acid process.

When the isopropyl alcohol is purified by conventional fractionation itis led to a heads column the purpose of which is to remove ether andother low-boiling waterinsoluble irnpurities, such as hydrocarbons,hydrocarbon polymers, oxygenated compounds, and unknown impurities. Thisis accomplished by a steam stripping operation whereby the lightermaterials are withdrawn overhead while weak aqueous alcohol is recoveredas bottoms. The weak aqueous alcohol is then distilled in aconcentrating column where the alcohol is concentrated to a compositionapproaching its water azeotrope, viz., 91 vol. percent isopropylalcohol-8 vol. percent water. The alcohol of azeotropic composition maybe further distilled for removal of acetone, if present, or concentratedfurther to 95 vol. percent or 99 vol. percent isopropyl alcohol, ifdesired.

When the crude isopropyl alcohol is being purified by extractivedistillation, the crude alcohol is distilled countercurrent to aninternal water reflux containing 70-99 mol percent water, preferably80-95 mol percent, whereby the low-boilingimpurities both water-solubleand waterayzaesa h t i 4 insoluble and some of the high-boilingimpurities are removed overhead, while a dilute aqueous solution of thealcohol is recovered as bottoms and sent to a concentrating column whereazeotropic alcohol is separated as a top sidestream from the balance ofthe high-boiling impurities and water.

It has been the practice in the commercial manufacture of isopropanol toemploy propane-propylene mixed gases which have been fractionated toremove all the C4 and higher hydrocarbons (down to a maximum below 0.2wt. percent) including parafiins both normal and iso, and olefins bothmonoand di-olefins. This procedure was adopted in order to minimize theamount of hydrocarbontype impurities giving rise to butyl odor in theresulting alcohol, which impurities were not all removable by theconventional distillation processes employed to purify the crudealcohol. With the advent of the application of extractive distillationemploying water as a solvent to the purification of aliphatic alcohols,and particularly isopropanol, it was found that these hydrocarbon-typeimpurities were completely removed and odors arising therefromeliminated from the alcohol, and at the same time extremely high yieldsof isopropanol and above) were obtained. However, water extractivedistillation, although highly effective for the recovery in high yieldsof isopropanol free of malodorous impurities giving rise tohydrocarbon-type or butyl odor, is ineffective for the complete removalfrom isopropanol of those light-boiling sulfur-bearing materials givingrise to mercaptan or recycle odor. Nor is the latter type odor removedby repeated distillations of the isopropanol.

It has been found that mercaptan type or recycle odor may be destroyedand isopropanol produced in high yields by incorporating into thepropylene stream to the sulfuric acid absorption operation 0.2 or 0.5 to5 wt. per cent of a C4 to C6 mono-olefin, preferably about 0.2 to 1.0wt. per cent of a C r-mouo-olefin, based on the weight of the totalhydrocarbon in the gas. It is believed that the C4 to C6 mono-olefinreacts with the low-boiling, low molecular Weight mercaptans,thioaldehydes or the thio-ketones or other sulfur-bearing compoundsconverting them to higher molecular weight materials which more closelyresemble the hydrocarbon-type impurities giving rise to butyl odor, andwhich are removed completely by Water extractive distillation.

Itis believed that recycle odor has its genesis in the formation of H28by the reaction of sulfuric acid with propylene. Hydrogen sulfide thenenters into a series of reactions producing low-boilingsulfur-containing compounds. The mechanism of the reactions ispostulated as follows:

H2804 CsHo Hi8, etc.

Has CaHg CHz-CH-SH-CH3 capable of being segregated by extractivedistillation from the alcohol, e. g.,

amass on. en ,ttiti'n c e-c etc.

In the purification of the crud'e'isopropanol by water extractivedistillation the procedure described in the above mentioned Serial No.24,626 is employed. The method therein described for purifyingisopropanol is summarized as follows in accordance with the flow diagramshown in the drawing:

The crude isopropanol containing high boiling and low boiling impuritiesis subjected to a distillation operation in which the crude alcohol isfed through line 1 to a distillation tower 2 at a point below the top,preferably at about the mid-point of the tower, and in which water isfed through line 3 to the top of the tower or at a point near the topthereof in sufficient quantity to maintain a composition of 70 to 99mol. per cent water in the liquid phase in the column. Theseconcentrations correspond to approximately 35-96 vol. per cent water,preferably 55-96 vol. per cent. The water thus supplied is sufiicient toestablish relative volatility relationships whereby there is removedoverhead through line 5 from the dis tillation tower all orsubstantially all of the impurities contained in the crude alcohol,namely, the low boiling impurities and the high boiling impurities.Among the impurities are light boiling hydrocarbons, some of which boilas low as 30 C. In addition the higher boiling impurities comprising theso-called propyl oils or polymer oils boiling as high as 250 C. to 300C. are removed overhead. The dilute aqueous isopropanol product obtainedthrough line 4 as bottoms from the distillation, and containingpreferably -25 vol. per cent isopropanol, is led to a concentratingcolumn 6 wherein the isopropanol is concentrated and recovered in highyields through line 7.

The effect of the presence of the C4 to Ce mono-olefins on theelimination of mercaptan or recycle odor from isopropanol is reflectedin the following examples.

Example 1 A propane-propylene refinery stream containing traces to 0.2wt. per cent (maximum) of Ct-hydrocarbons ineluding mono-olefins wascontacted with 65-70 wt. per

cent H2804 at temperatures of 7080 C. in the manner previously outlinedfor the weak-acid process. The crude isopropanol was refined by both theconventional distillation procedure and by water extractivedistillation. The

refined alcohols in both cases were characterized by the presence ofrecycle odor rendering them unsaleable for many purposes, such assolvents, etc.

Example 2 The following data illustrate the process of the invention asapplied to a crude isopropanol prepared by absorbing a propylene streamin sulfuric acid of 70 wt. per cent strength followed by extractivedistillation of the product employing water as the extractive solvent.The data show that the propylene stream free of Ct-hydrocarbons resultedin a final alcohol product which was characterized by the presence ofrecycle odor," whereas a feed containing 0.36 wt. per centct-hydrocarbons produced a cards iiiiwlibifwlilcll when purified"hunter-extractive distillation-yielded apnnie truce "alcohol withers"recycle odor.

1 1 Run No 2 3 Production Rate, gab/S. D 45, 873 27,800 Odor rating.9.... wPrime (no ileguiar recycle (recycle odor) odor) Wt. percent OrinOr stream; 36 o Extractive Column Operation:

Crude Alcohol feed rate, G. P. H 3, 580 2, 060

Water feed to column, G. P. H 4, 330 5, 379 Extractive Products:

Vol. percent Ether in overhead 80. 0 40. 0

Vol. percent Alcohol in overhead l. 0 l1. 0

Vol. percent Alcohol in Water Extract Bottoms 22. 0 16. 0

Finishing Column:

Feed rate, G. P. H 8, 500 8, 000

Sidestream rate, G. P. H 345 110 Reflux rate, G. P. H 8, 500 7,800

Steam rate, G. P. H 32,000 28, 000

Example 4 The following runs were carried out on crude alcohol preparedby absorbing a propylene stream in an acid of wt. percent strength. Thepropylene stream contained 0.26 wt. percent of C4 hydrocarbons. Theresulting crude isopropanol was purified by water extractivedistillation and an alcohol of prime grade free of recycle odor" wasHaving descibed the invention in a manner so that it may be practiced bythose skilled in the art what is claimed is:

1. In a process of producing isopropanol by hydration of a propylenehydrocarbon feed which contains impurities normally giving rise tomercaptan odor and also contains less than 0.2 percent of C4 to Camono-olefinic hydrocarbons, the improvement of minimizing mercaptan odorwhich comprises adding to said propylene feed enough of a C4 to Camono-olefinic hydrocarbon to pro duce a modified propylene feedcontaining 0.5 to. 5 weight percent of said C4 to C6 mono-olefinichydrocarbon, absorbing said modified propylene feed in sulfuric acid of60 to 95 weight percent strength to produce an acid extract, hydrolyzingthe extract to produce crude isopropanol containing low boilingimpurities and simultaneously reacting the C4-C6 olefin with theodor-causing impurities to convert them to high boiling impuritiesboiling as high as 250-300" C., introducing the crude isopropanol into apoint near the mid-section of a distillation zone, introducing water toan upper portion of the distillation zone in amounts sufficient tomaintain a water concentration of 70 to 99 mol percent in the liquidphase in the distillation zone, applying heat to the distillation zoneto distill therefrom vapors which rise countercur rently to thedownflowing aqueous reflux, removing overhead from the distillation zonelow-boiling and high-boiling impurities including impurities normallygiving rise to mercaptan odor and removing from the bottom of saiddistillation zone a dilute aqueous isopropanol substantially free ofmercaptan odor.

2. A process according to claim 1 in which the water concentration inthe distillation zone is maintained in the range of to mol percent.

Z 3. A process according to tilaim 1. in which the water is removed fromthe dilute isopropanol byconcentration in a stripping zone. j 4. Theprocess of claim 1 in which the C4-C6 olefinic' hydrocarbon is aC4mono-o1efin in an amount of about 5;

0.5 to 1.0 weight per eent.

2,081,111 Bump May 11, 1957 Burk et a1. -1" Apr. 9, 1940 Metzl T July21, 1942 Egb'erts et al.- June 6, 1950 Gilliland et a1 May 8, 1951'Beddow Feb. 19, 1952 Drout, Jr Sept. 9, 1952 FOREIGN PATENTS GreatBritain Jan. 28, 1940

1. IN A PROCESS OF PRODUCING ISOPROPANOL BY HYDRATION OF A PROPYLENEHYDROCARBON FEED WHICH CONTAINS IMPURITIES NORMALLY GIVING RISE TOMERCAPTAN ODOR AND ALSO CONTAINS LESS THAN 0.2 PERCENT OF C4 TO C6MONO-OLEFINIC HYDROCARBONS, THE IMPROVEMENT OF MINIMIZING MERCAPTAN ODORWHICH COMPRISES ADDING TO SAID PROPYLENE FEED ENOUGH OF A C4 TO C6MONO-OLEFINIC HYDROCARBON TO PRODUCE A MODIFIED PROPYLENE FEEDCONTAINING 0.5 TO 5 WEIGHT PERCENT OF SAID C4 TO C6 MONO-OLEFINICHYDROCARBON, ABSORBING SAID MODIFIED PROPYLENE FEED IN SULFURIC ACID OF60 TO 95 WEIGHT PERCENT STRENGTH TO PRODUCE AN ACID EXTRACT, HYDROLYZINGTHE EXTRACT TO PRODUCE CRUDE ISOPROPANOL CONTAINING LOW BOILINGIMPURITIES AND SIMULTANEOUSLY REACTING THE C4-C6 OLEFIN WITH THEODOR-CAUSING IMPURITIES TO CONVERT THEM TO HIGH BOILING IMPURITIESBOILING AS HIGH AS 250-300* C., INTRODUCING THE CRUDE ISOPROPANOL INTO APOINT NEAR THE MID-SECTION OF A DISTILLATION ZONE, INTRODUCING WATER TOAN UPPER PORTION OF THE DISTILLATION ZONE IN AMOUNTS SUFFICIENTS TOMAINTAIN A WATER CONCENTRATION OF 70-99 MOL PERCENT IN THE LIQUID PHASEIN THE DISTILLATION ZONE, APPLYING HEAT TO THE DISTILLATION ZONE TODISTILL THEREFROM VAPORS WHICH RISE COUNTERCURRENTLY TO THE DOWNFLOWINGAQUEOUS REFLUX, REMOVING OVERHEAD FROM THE DISTILLATIN ZONE LOW-BOILINLGAND HIGH-BOILING IMPURITIES INCLUDING IMPURITIES NORMALLY GIVING RISE TOMERCAPTAN ODOR AND REMOVING FROM THE BOTTOM OF SAID DISTILLATION ZONE ADILUTE AQUEOUS ISOPROPANOL SUBSTANTIALLY FREE OF MERCAPTAN ODOR.